A tandem-type image forming apparatus is well known in the art. One such tandem-type image forming apparatus has photosensitive drums corresponding to each of the colors yellow, magenta, cyan, and black that are juxtaposed in a horizontal direction. In this tandem-type image forming apparatus, toner images in each color are formed almost simultaneously on the respective photosensitive drums and are then sequentially transferred from the photosensitive drums to a sheet of paper conveyed sequentially past each photosensitive drum so that the images are superimposed on one another, thereby achieving color image formation at substantially the same speed as single-color image formation with a monochromatic image forming apparatus.
However, this tandem-type color image forming apparatus can have problems with color registration when the color toner images are not properly aligned. For example, if a photosensitive drum has eccentricity, the distance between a scanning unit and the exposed surface of the photosensitive drum varies depending on the rotated position of the photosensitive drum, producing error in the laser scanning width on the photosensitive drum. When such error varies for each photosensitive drum, the toner images in each color cannot be properly aligned on the paper, resulting in color registration error.
Technologies, such as that disclosed in Japanese Patent Application Publication No. 2001-188395, have been proposed for reducing color registration error by canceling the effects of variations in the distance between the scanning unit and the exposure surface of the photosensitive drum caused by eccentricity of the drum. This is achieved by detecting the eccentric phase of each photosensitive drum and adjusting the speed of the motor driving each photosensitive drum to match the exposure start timing (write out timing of the electrostatic latent image) for each photosensitive drum so that the eccentric phase of each photosensitive drum is aligned relative to the image.
However, the conventional method described above requires such components as sensors for detecting the eccentric phase of the photosensitive drums and circuits for processing the detection signals outputted by the sensors, thereby increasing the complexity of the structure of the image forming apparatus as well as manufacturing costs.